1. Field of the Invention
This invention relates generally to intravenous (W) fluid warming devices and particularly, a warming system used for warming of IV fluids prior to introduction into a body and more particularly, to an IV fluid warming system having a presence detector to detect the presence of an IV fluid warming cassette in a warming unit.
2. Description of the Related Art
Intravenous fluid infusion is a commonly used clinical technique. Since the infused fluids (also xe2x80x9cIV fluidsxe2x80x9d) are usually stored at cool temperatures to preserve freshness, they must frequently be heated before introduction into a body. For infusion of fluids into a human it may be desirable to raise the temperature of the fluid to a normal core body temperature of about 98.6 F. In other cases, other temperatures may be indicated. For example, during open heart surgery the temperature of patient is lowered to a hypothermic level; fluid must therefore be infused at the same lower temperature.
The prior art embraces systems for warming fluids as they are being infused into a body. Such systems, which may be denoted as xe2x80x9cfluid warmingxe2x80x9d systems, have utilized a variety of means for heating fluids. Such means include heating by conduction or convection, with heat being provided by a heated fluid such as air or from an electro-resistive source such as a coil or plate. There are problems that are particular to each of these systems, especially in the clinical context. For example, one system heats fluid to be infused by conducting it through a heated fluid. Such systems typically are heavy, cumbersome, require frequent cleaning, and can pollute the clinical environment, where cleanliness is of vital importance. Typically these systems dispose a conduit in a dense fluid such as water, heat the water, and conduct the fluid to be infused through the conduit, relying upon heat to be transferred by conduction from the heated water, through the conduit to the fluid. Such systems rely upon a reservoir to contain a constant volume of heated water. This reservoir can become contaminated and proliferate undesirable bacterial agents. Therefore leaks in such systems are of particular concern in sterile settings.
In other systems, heat is transferred from an electro-resistive heating element to the fluid which is contained in a heat exchanger structure that provides a fluid pathway for the fluid to travel and a conductive pathway for thermal energy to be transferred from the heating element to the fluid. One example is an in-line fluid heating apparatus that includes an enclosure containing one or more heating elements and a cassette that is removeably received within the enclosure. The cassette defines a complex fluid flow pathway. The outline of the fluid flow pathway is preferably precisely replicated in the heating elements of the enclosure in order to maximize the xe2x80x9cdwell timexe2x80x9d of the fluid in the pathway thereby to maximize the potential amount of heat transferred to the fluid as it flows through the pathway in the cassette. These systems are termed xe2x80x9cdry heat warmingxe2x80x9d systems.
Dry heat warming systems are, at this point, preferred for heating fluid to be infused. However, the dry heat warming systems that are available tend to exhibit suboptimal performance for a number of reasons. Clinical practice today indicates the desirability of providing fluid flow for intravenous infusion in a broad range of rates, from a rate sufficient to keep a vein open (KVO) up to 30,000 ml/hr. Manifestly, the transfer of thermal energy to the fluid must keep pace with the flow rate of the fluid; heat transfer must take place rapidly to heat a fluid in a high volume, rapid infusion situation. However, the rate of heating must be carefully matched to the rate of fluid flow One significant drawback of prior art dry heat warming systems is a mismatch between the rate of heating and the rate of fluid flow; sometimes the fluid is heated too rapidly, resulting in temperatures well above a desired temperature. Such over heating can damage fluids, particularly blood. Overheated blood produces hemolysis, the disintegration of red blood cells. Manifestly, the fluid warming system must well calibrate the rate of fluid flow to the rate of heating.
Calibration of fluid heating with respect to fluid flow depends on many parameters that are inherent in the construction of an insertable heat exchanger, in the shape of the fluid flow path of the heat exchanger, and in the positioning of the heat exchanger in the warming unit. Particularly, misregistration between the fluid flow path of the heat exchanger and the corresponding shape of a heating element can result in undesirable temperatures outside of a predetermined temperature range. For example, a heater plate in a warming unit might be configured in such a manner as to vary the rate at which heat is conducted to the fluid pathway. In this regard, the heater might deliver a greater amount of heat at the inlet of the fluid pathway than at the outlet. In such a case, an accidental reversal of the heat exchanger in the heating unit would almost certainly result in improper heating of the fluid. Similarly, if control of the warming unit depends upon a heat sensing element disposed at a particular location with respect to the heat exchanger, accidental reversal could result in a erroneous control of the warming unit and improper heating of the fluid. Such reversal is entirely possible in the case of cassette that may inserted into and removed from a warming unit.
Typically, dry heat fluid warming systems capable of heating hydrating fluids within a broad temperature range may be burdened with sophisticated and expensive functional and mechanical hardware to ensure proper operation. In a warming system where the design and construction of a removable heat exchanger are precisely optimally matched to the design, construction, and performance of the warming unit, any provision to ensure proper orientation between the heat exchanger and the warming unit would improve the efficiency, safety, and cost of the system.
From the discussion above, it should be apparent that there is a need for an in-line IV fluid warming system of the type including a removable heat exchanger and a warming unit that can heat IV fluids quickly, efficiently and consistently, without damaging the fluid, for immediate and safe use with a patient. Importantly, such a system should guarantee correct alignment between the heat exchanger and heating elements in the warming unit. This invention satisfies these needs.
Broadly, the present invention concerns the warming of an IV fluid during infusion into the body of a person or animal: Typically, IV fluid (including blood) is stored at low temperatures to prolong its freshness. Before use, it must be warmed. During emergencies and certain surgical procedures, the fluid must be warmed quickly. The present invention allows the IV fluid to be warmed in line as it flows from an IV reservoir to a person.
This invention is an intravenous (IV) fluid warming system having a warming unit that receives an insertable heat exchanger, preferably embodied as a cassette. A presence detection circuit renders the warming unit inoperative when the cassette is not in place, when the cassette is inserted incorrectly, or when an incompatible cassette is present.
In a preferred embodiment, the warming unit comprises an enclosure supporting a heater plate assembly. The heater plate assembly has an opening inlet into which the cassette may be inserted. The heater plate assembly includes a first heater plate positioned on one side of the inlet and a second heater plate positioned on an opposing side of the inlet, such that when the cassette is positioned in the warming unit, the first heater plate is positioned on one side of the cassette and the second heater plate is positioned on an opposing side of the cassette. Operation of the heater plate assembly is enabled in response to an indication by the presence detection circuit that the cassette has been correctly seated in the inlet. In an illustrative example of the presence detection circuit, a magnet is located on or in the first heater plate and a sensor is located on or in the second heater plate so as to be able to detect the magnet, the magnet and sensor being separated by the width of the inlet slot. A presence indicator is positioned on a cassette such that when the cassette is properly inserted into the warming unit, the presence indicator is disposed between the magnet and the sensor, disabling the sensor with respect to the magnet, and enabling the warming unit to operate the heater plates.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiments, which illustrate, by way of example, the principles of the invention.